/**
 * 6. Z 字形变换
 * https://leetcode.cn/problems/zigzag-conversion/description/
 */
class Solution {
    public String convert(String s, int numRows) {
        //处理边界情况
        if(numRows == 1) return s;
        StringBuilder ret = new StringBuilder();
        int d = numRows * 2 - 2, n = s.length();
        //处理首行
        for(int i = 0; i < n; i += d) {
            ret.append(s.charAt(i));
        }
        //处理中间行
        for(int k = 1; k < numRows - 1; k++) {
            for(int i = k, j = d - k; i < n || j < n; i += d, j += d) {
                if(i < n) ret.append(s.charAt(i));
                if(j < n) ret.append(s.charAt(j));
            }
        }
        //处理最后一行
        for(int i = numRows - 1; i < n; i += d) {
            ret.append(s.charAt(i));
        }
        return ret.toString();
    }
}